This invention relates to laser processing systems and methods, including systems and methods for removing, with high yield, closely-spaced metal link structures or xe2x80x9cfusesxe2x80x9d on a silicon substrate of an integrated circuit or memory device.
Laser systems can be employed to remove fuse structures (xe2x80x9cblow linksxe2x80x9d) in integrated circuits and memory devices such as ASICs, DRAMs, and SRAMs, for purposes such as removing defective elements and replacing them with redundant elements provided for this purpose (xe2x80x9credundant memory repairxe2x80x9d), or programming of logic devices. Link processing laser systems include the M320 and M325 systems manufactured by General Scanning, Inc, which produce laser outputs over a variety of wavelengths, including 1.047 xcexcm, 1.064 xcexcm, and 1.32 xcexcm.
Economic imperatives have led to the development of smaller, more complex, higher-density semiconductor structures. These smaller structures can have the advantage of operation at relatively high speed. Also, because the semiconductor device part can be smaller, a greater number of parts can be included in a single wafer. Because the cost of processing a single wafer in a semiconductor fabrication plant can be almost independent of the number of parts on the wafer, the greater number of parts per wafer can translate into lower cost per part.
In the 1980s, semiconductor device parts often included polysilicon or silicide interconnects. Although poly-based interconnects are relatively poor conductors, they were easily fabricated using processes available at the time, and were well-suited to the wavelengths generated by the Nd:YAG lasers commonly available at the time. As geometries shrank, however, the poor conductivity of polysilicon interconnects and link structures became problematic, and some semiconductor manufacturers switched to aluminum. It was found that certain conventional lasers did not cut the aluminum links as well as they had cut polysilicon links, and in particular that damage to the silicon substrate could occur. This situation could be explained by the fact that the reflection in aluminum is very high and the absorption is low. Therefore, increased energy must be used to overcome this low absorption. The higher energy can tend to damage the substrate when too much energy is used.
Sun et al., U.S. Pat. No. 5,265,114 advances an xe2x80x9cabsorption contrastxe2x80x9d model for selecting an appropriate laser wavelength to cut aluminum and other metals such as nickel, tungsten, and platinum. In particular, this patent describes selecting a wavelength range in which silicon is almost transparent and in which the optical absorption behavior of the metal link material is sufficient for the link to be processed. The patent states that the 1.2 to 2.0 xcexcm wavelength range provides a high absorption contrast between a silicon substrate and high-conductivity link structures, as compared with laser wavelengths of 1.064 xcexcm and 0.532 xcexcm.
The invention provides a system and method for vaporizing a target structure on a substrate. According to the invention, a calculation is performed, as a function of wavelength, of an incident beam energy necessary to deposit unit energy in the target structure. Then, for the incident beam energy, the energy expected to be deposited in the substrate as a function of wavelength is calculated. A wavelength is identified that corresponds to a relatively low value of the energy expected to be deposited in the substrate, the low value being substantially less than a value of the energy expected to be deposited in the substrate at a higher wavelength. A laser system is provided configured to produce a laser output at the wavelength corresponding to the relatively low value of the energy expected to be deposited in the substrate. The laser output is directed at the target structure on the substrate at the wavelength corresponding to the relatively low value of the energy expected to be deposited in the substrate, in order to vaporize the target structure.
Certain applications of the invention involve selection of a wavelength appropriate for cutting a metal link without producing unacceptable damage to a silicon substrate, where the wavelength is less than, rather than greater than, the conventional wavelengths of 1.047 xcexcm and 1.064 xcexcm. This method of wavelength selection is advantageous because the use of shorter wavelengths can result in smaller laser spots, other things being equal, and hence greater ease in hitting only the desired link with the laser spot. In particular, other things being equal, laser spot size is directly proportional to wavelength according to the formula: spot size is proportional to xcexf, where xcex is the laser wavelength and f is the f-number of the optical system.
Moreover, certain applications of the invention involve selection of a wavelength at which a substrate has low absorption but an interconnect material has higher absorption than at conventional wavelengths of 1.047 xcexcm and 1.064 xcexcm or higher-than-conventional wavelengths. Because of the reduced reflectivity of the interconnect material, the incident laser energy can be reduced while the interconnect material nevertheless absorbs sufficient energy for the interconnect to be blown without multiple laser pulses (which can impact throughput) or substantial collateral damage due to the laser beam.
The invention can effect high-quality laser link cuts on high-conductivity interconnect materials such as copper, gold, and the like, arranged in closely-spaced patterns, with only a single laser pulse, and without damaging the substrate. The invention can further allow a smaller laser spot size than would be obtainable at wavelengths of 1.047 xcexcm, 1.064 xcexcm, or higher, while still providing acceptable link cuts.